Add gyp file.

GYP is a project generator heavily used by node.js and Chromium, adding the gyp file makes it easy to integrate libgit2 into these projects. This commit is mostly for node.js binding to libgit2.

Right now I only tested static library compilation on Linux, some commands in `CMakeLists.txt` have not been implemented yet, I'll keep improving it.

I can take the future maintaining of the gyp file, when you change compilation parameters or add/remove source files, you can ping me and I'll update the gyp file.